The present invention relates to a thermal printer having a flat platen support mechanism, more particularly to a non-impact type thermal printer such as a thermal printer having a flat platen support mechanism and a thermal transfer printer having a flat platen support mechanism etc.. The present invention relates to a thermal printer having an improved flat platen support mechanism suitable for a uniform face contact between a platen printing flat face of the flat platen and a printing face of a thermal head.
A conventional thermal printer or thermal transfer printer employs in general a cylindrical platen (a platen having a round cross-sectional shape or a round platen) therein. However, this round platen has a comparatively large outer diameter dimension and has to form a flat portion owing to the elastic deformation at a contact portion against a thermal head. Therefore, the conventional thermal printer having the round platen or the thermal transfer printer having the round platen has a fatal defect therein, namely it is necessary to give a comparative large head pressing force against the thermal head.
On the other hand, a thermal printer including a flat platen having a flat portion or a thermal transfer printer including a flat platen having a flat portion does not require the elastic deformation for the flat platen. It makes the head pressing force of the thermal head against the flat platen smaller than that of the round platen, and also the flat platen becomes smaller in size than that of the round platen. Therefore the miniaturization of the printing apparatus is made possible by employing the flat platen.
The conventional thermal printer having the flat platen or the thermal transfer printer having the flat platen can achieve a high head contact face pressure, even under same head pressing force, in comparison with the thermal printer having the round platen or the thermal transfer printer having the round platen. The thermal printer having the flat platen or the thermal transfer printer having the flat platen can thus achieve a high printing quality on even a low smoothness transfer printing paper.
However, with the use of the thermal printer having the flat platen or the thermal transfer printer having the flat platen, the greatest technical problem to be solved therein is a head contact problem for the platen printing flat face of the flat platen against the printing face of the thermal head.
The contact condition between the conventional round platen and the thermal head is shown in FIG. 14. When a thermal head 101 contacts with a round platen 102 made of a rubber, even the thermal head 101 may contact through a transfer printing paper 103 to the round platen 102 under the inclination condition with a very small angle .theta..sub.o as shown in FIG. 14. A flat portion l.sub.o of the round platen 102 can be formed anywhere at the circumference of the round platen 102 so as to correspond to such situation. When the round platen 102 is employed in the thermal printer or the thermal transfer printer, the uniform head contact can be obtained always thereon.
On the other hand, the contact condition between the conventional flat platen and the thermal head is shown in FIG. 15. When a thermal head 101 contacts a flat platen 104, which is made of a rubber, and supported by a flat platen support plate 105, the thermal head 101 may contact through a transfer printing paper 103 with the flat platen 104 under the inclination condition with a very small angle .theta..sub.o as shown in FIG. 15. Only a part of the platen printing flat face 104a of the flat platen 104 is contacted by the printing face of the thermal head 101. However, it is impossible for the printing face of the thermal head 101 to contact uniformly with the whole platen printing flat face 104a of the flat platen 104. This causes a fatal defect because of occurrence of a printing deficiency.
From the experimentation by the inventors of the present invention, when the rubber platen having a rubber hardness H.sub.s =70.degree. (JIS A scale) is employed in the thermal printer having the flat platen or the transfer thermal printer having the flat platen, it is necessary to keep a very small angle .theta..sub.o below about .+-.0.1.degree. so as to contact wholly the thermal head 101 with the platen printing flat face 104a of the flat platen 104. However it is impossible to keep within the allowed value of the very small angle .theta..sub.o as a practical matter in respect to the measurement error for installing the thermal head 101 and the flat platen 104.
Further, the case in which the flat platen 104 having a very small rubber hardness such as below H.sub.s =20.degree. is employed and elastically deforms so as to contact wholly with the platen printing flat face 104a of the flat platen 104 is shown in FIG. 16. In this case, the reaction force distribution generated in the flat platen 104 becomes non-uniform at the upper side and the lower side of the flat platen 104 as shown in FIG. 17, because the deformation amount of the flat platen 104 differs at the upper side and the lower side thereof.
Such a non-uniformness of the reaction force distribution causes uneven printing density. Further, in the thermal transfer printer using an ink ribbon, non-uniformness causes a raising (shift up) phenomenon and a lowering (shift down) phenomenon of the ink ribbon. Such an ink ribbon shift up phenomenon and an ink ribbon shift down phenomenon is defined as a slip phenomenon, which is an ink ribbon slip movement toward the upper direction of the lower direction accompanying the transverse travelling of the thermal head by the ink ribbon movement from the large flat platen reaction force side to the small flat platen reaction force side.
In order to solve the above stated problems, a mechanism for uniform face contacting of the platen printing flat face of the flat platen with the printing face of the thermal head is proposed in, for example, Japanese Patent Laid-Open Nos. 201879/1984 and 56877/1983 etc.
Japanese Patent Laid-Open No. 201879/1984 discloses a thermal transfer printer wherein a triangular groove, which is provided at the central portion of the rear side of a flat platen support plate along the longitudinal direction, engages with a triangular projection. The triangular projection is provided on a frame body under the condition maintaining backlash condition. The flat platen support plate can swing freely about the apex of the triangular projection.
Japanese Patent Laid-Open No. 56877/1983 discloses a thermal transfer printer wherein support portions are provided on both sides of a flat platen. The support portion is supported movably freely with a frame body. A contact portion is provided on the rear side of the flat platen so as to contact with frame body. The center of rotation for the flat platen according to the support portion is consistent with the center of rotation of the contact portion. With the above stated thermal transfer printer, however, it is unable to obtain a uniform face contact between the thermal head and the flat platen.
The inventors of the present invention have been studied the representative conventional prior art as shown in FIG. 18, which modified the structure for the flat platen support mechanism of the thermal transfer printer shown in Japanese Patent Laid-Open No. 201879/1984.
A thermal head 101 is supported on a head holder 106 as shown in FIG. 18. The head holder 106 is supported so as to swing freely about the center point, which is a center 107a of a carriage transverse shaft 107. A flat platen 104 is supported integrally with a flat platen support plate 108.
The flat platen support plate 108 provides a triangular groove 108a having an angle about 90.degree. at the central portion of the rear side along the longitudinal direction. The flat platen support plate 108 is supported through a frame body 109. The frame body 109 provides a triangular projection 109a having an angle about 60.degree. along the longitudinal direction. A vertex 108b of the triangular groove 108a engages with a vertex 109b of the triangular projection 109a. The whole flat platen support plate 108 can swing freely about the vertex 108b of the triangular groove 108a.
The thermal head 101 presses through a transfer printing paper 103 to the platen printing flat face 104a of the flat platen 104 with a relative angle .theta..sub.o. The thermal head 101 is pressed by the head holder 106 with a head pressing force W.sub.o.
A contact point of the thermal head 101 against the transfer printing paper 103 is at a point P.sub.o which corresponds to the corner portion of the flat platen 104. In order to face contact the platen printing flat face 104a of the flat platen 104 against a printing face 101a of the thermal head 101, it is necessary that the angle of the flat platen 104 be the same angle of the thermal head 101 for the swing movement of the flat platen 104.
The flat platen 104 is unable to swing without the relative slip movement between the transfer printing paper 103 and the thermal head 101. The friction resistance force against the slip movement exists between the transfer printing paper 103 and the thermal head 101. Therefore the friction resistance force becomes a large resistance force against the swing movement of the flat platen 104. The restriction points of the whole flat platen 104 appear at two places which are the vertex 108b of the triangular groove 108a and the point P.sub.o. Therefore no swing movement of the flat platen 104 occurs therein. Namely, it is impossible to obtain the uniform face contact between the thermal head 101 and the flat platen 104 in this thermal transfer printer.
Further, the inventors of the present invention studied the conventional prior art as shown in FIG. 19, which modified the structure for the flat platen support mechanism of the thermal transfer printer shown in Japanese Patent Laid-Open No. 56887/1983.
A thermal head 101 is supported by a head holder 106 as shown in FIG. 19. The head holder 106 is supported so as to swing freely about the center point 107a of a carriage transverse shaft 107. A projection portion 110a having a curvature R is provided on the rear side of a flat platen 110 along the longitudinal direction of the flat platen 110. The projection portion 110a of the flat platen 110 contacts a frame body 111.
Two support pins 110b are provided on both end sides of the flat platen 110. A vertex 110c of the support pin 110b is consistent with the center of curvature of the projection portion 110a. The support pin 110b is supported so as to swing freely about a support pin receiving portion 111a, which is provided on the frame body 111. The flat platen 110 swing freely about the vertex 110c of the support pin 110b.
The printing face 101a of the thermal head 101 is pressed through the transfer printing paper 103 against the platen printing flat face 104a of the flat platen 110 with a relative angle .theta..sub.o. The restriction points of the whole flat platen 110 appear at two places, which are the vertex 110c of the support pin 110b and the point P.sub.o.
Therefore no swing movement of the flat platen 110 occurs therein. Namely, it is impossible to obtain the uniform face contact between the platen printing flat face 104a of the flat platen 110 and the printing face 101a of the thermal head 101 in this thermal transfer printer.